1. Field of the Invention
This invention relates to the continuous casting of steel and more particularly to the use of a sealed tundish in the continuous casting of steel, and to an improved tundish seal for such use.
2. Description of the Prior Art
The adverse effects of atmospheric oxygen and nitrogen contacting molten steel in a pouring or casting operation are well known, and numerous patents have issued on apparatus and devices for shielding the molten metal from the atmosphere to prevent oxidation and nitrogen pickup during such operations. In a continuous casting operation, it is conventional for the molten steel to be transported from the convertor to the caster in a ladle where it is supported on a turret for movement into position above a tundish having its pouring outlet, or outlets, directly above the open top of the caster mold, or molds.
Molten steel in the ladle is covered with a layer of slag, and is dispensed through an outlet in the ladle bottom under control of a slide valve. A refractory shroud tube attached to the slide valve extends through an opening in the tundish cover to a point beneath the surface of the pool of metal in the tundish during casting. From the tundish, the metal flows through one or more bottom outlets under control of stopper valves, again through a refractory tube or tubes extending below the level of the molten metal in the caster mold. Thus, the molten steel is shielded from the atmosphere in its flow path from the ladle into the tundish, and from the tundish into the mold. In the tundish, however, and particularly during the initial filling of a newly lined tundish, contact with the atmosphere can result in substantial nitrogen pickup and oxidation of the molten steel. Indeed, in casting of high purity prime priced steels suitable for forming exposed automobile body components or for tinplate, it has in the past been generally necessary to downgrade or even scrap up to 5 or 6 slabs, each weighing up to 30,000 pounds or more, cut from the leading end of a continuous strand cast using a newly lined tundish.
Attempts have been made to purge atmosphere containing free oxygen and nitrogen from the interior of a tundish by injecting an inert gas into the tundish, one such continuous casting system being illustrated in U.S. Pat. No. 3,558,256; however, because of the size of the tundish used in modern casters, and the number and size of openings required in the conventional tundish cover, such attempts to purge the tundish generally have required an excessive amount of inert gas and have not been entirely successful. Further, in many tundishes in use today, particularly for multi-strand casters, the cover itself is formed in a number of sections having openings therethrough as for the pouring of steel from the ladle or to accommodate the insertion and manipulation of the stopper rods employed to control the flow rate from the tundish.
Dimensional tolerances in commercial continuous casting machines are such that openings in the tundish cover are necessarily substantially larger than the shroud tube or stopper rod to be inserted therethrough. Unavoidable spacing or cracks between adjacent tundish cover sections and between the cover sections and the top of the tundish, and the opening in the sidewall for a dumping spout provide additional openings to the interior of the tundish. Attempts to purge such prior art covered tundishes have therefore been only marginally effective.
U.S. Pat. No. 5,368,208 discloses a further attempt to shield air from molten metal in a continuous casting facility and illustrates, in the drawings, a large opening into the tundish in the form of a pouring or dumping spout. No attempt is made to seal the top of the tundish beneath the cover.
U.S. Pat. No. 3,459,346 discloses a molten metal pouring spout employed between a ladle and a tundish and illustrates the pouring spout projecting downwardly through the cover on the tundish. FIG. 1 illustrates the spout extending through the opening in the tundish cover with substantial clearance as is considered necessary in such operations.
It is, accordingly, a primary object of the present invention is to provide an improved seal between the open top of the tundish and the rigid tundish cover to substantially eliminate the ingress of atmospheric air into the tundish by maintaining a positive purging gas pressure in the tundish during the casting operation.
Another object is to provide such a tundish seal which enables efficient and economical purging and pressurizing of the tundish space with an inert gas to effectively eliminate free oxygen and nitrogen from the tundish prior to and during the casting operation.
Another object is to provide a lightweight, economical and efficient, disposable seal which may be quickly and easily positioned over the open top of the tundish before placing the cover thereon, with the seal material extending over the openings provided in the tundish cover and being penetrable in the areas of such openings by the ladle shroud tube and tundish flow control stopper rods.